Inverse Kinematics Documentation
I do not know of any books on this specific to blender.

Developers rarely write books on the details of particular implementations, and for blender, any implementation documentation outside the code base will most likely be found on the wiki. Details of techniques and algorithms are generally best studied from the original paper(s). 

As mentioned, blender is open source, so you can read the source code in git yourself.

Skimming it, it looks like the wiki is actually fairly detailed here. It also mentions the original paper which may be helpful, if you have access.

Since iTAsC was originally designed for robot control systems, you may want to look at literature in that area.